The present invention relates to a method of fabricating a composite material part comprising fiber reinforcement densified by a matrix.
More particularly, but not exclusively, the field of application of the invention relates to thermostructural composite materials, i.e. to composite materials having good mechanical properties and a capacity for conserving these properties at high temperatures. Typical thermostructural materials are carbon/carbon (C/C) composite materials formed by carbon fiber reinforcement densified by a carbon matrix, and ceramic matrix composite (CMC) materials formed by refractory fiber reinforcement (carbon fibers or ceramic fibers) densified by a matrix that is at least partially ceramic. Examples of CMCs are C/SiC composites (carbon fiber reinforcement and silicon carbide matrix), C/C—SiC composites (carbon fiber reinforcement and matrix comprising a carbon phase, generally next to the fibers, and a silicon carbide phase), and SiC/SiC composites (reinforcing fibers and matrix made of silicon carbide). An interphase layer may be interposed between the reinforcing fibers and the matrix in order to improve the mechanical strength of the material.
Fabricating a thermostructural composite material part generally comprises making a fiber preform of shape close to the shape of the part that is to be fabricated, and densifying the preform with the matrix.
The fiber preform constitutes the reinforcement of the part and it performs a role that is essential in terms of mechanical properties. The preform is obtained using fiber fabrics: yarns, tows, braids, woven cloth, felts, . . . . Shaping is performed by winding, weaving, stacking, and possibly needling two-dimensional plies of cloth or of sheets of tows.
Densifying the fiber preform consists in filling in the pores in the preform throughout all or some of its volume, using the material that constitutes the matrix.
The matrix of a composite material may be obtained by using various known methods and in particular by using a liquid technique or using a gaseous technique.
The method using a liquid technique consists in impregnating the preform with a liquid composition containing an organic precursor for the material of the matrix. The organic precursor is usually in the form of a polymer, such as a resin, and it is optionally diluted in a solvent. The precursor is transformed into a refractory phase by heat treatment, after eliminating any solvent and after curing the polymer. The heat treatment consists in pyrolyzing the organic precursor in order to transform the organic matrix into a matrix of carbon or of ceramic depending on the precursor used and on pyrolysis conditions. By way of example, liquid precursors for carbon may be resins having a relatively high coke content, such as phenolic resins, whereas liquid precursors for ceramic, in particular for SiC, may be resins of the polycarbosilane (PCS) type, of the polysiloxane (PSX) type, of the polytitanocarbosilane (PTCS) type, or of the polysilazane (PSZ) type. A plurality of consecutive cycles running from impregnation to heat treatment may be performed in order to achieve the desired degree of densification.
The method using a gaseous technique consists in chemical vapor infiltration (CVI). The fiber preform is placed in an oven into which a reaction gas phase is admitted. The pressure and the temperature that exist in the oven and the composition of the gas phase are selected so as to enable the gas phase to diffuse within the pores of the preform in order to form the matrix therein by depositing solid material, in contact with the fibers, which solid material results from decomposition of an ingredient of the gaseous phase or from a reaction between a plurality of its ingredients. For example, gaseous precursors for carbon may be hydrocarbons that produce carbon by cracking, such as methane, and a gaseous precursor for ceramic, in particular for SiC, may be methyltricholorosilane (MTS) giving SiC by decomposition of the MTS (possibly in the presence of hydrogen).
In order to obtain good and uniform densification of the fiber preform and thereby confer good mechanical properties to the part, the matrix must be deposited not only in the pores that are present between the yarns of the preform (inter-yarn pores or spaces), but also in the pores that are present within the yarns, i.e. in the pores between the filaments of a given yarn (intra-yarn pores or spaces).
Nevertheless, it is often difficult to achieve densification to the core of a continuous yarn, whether by a liquid technique or by a gaseous technique, because the mean distance between the fibers in the yarns is too small, and consequently because inter-yarn pores are too small and difficult to access from outside the yarns, in particular when the fabric is made by weaving. Under certain circumstances, too small a quantity of matrix within the yarns can lead to a reduction in the mechanical properties and in the fatigue strength of the material under stress at high temperature.
Document U.S. Pat. No. 5,217,796 describes making printed circuit cards from a fiber fabric constituted by inorganic fibers, e.g. glass fibers, the fabric being reinforced with a resin. In that document, the fabric is subjected to jets of water under pressure in order to open up the yarns that are exposed on the surface of the fabric. Nevertheless, in that document, only the surface yarns are treated and the pressure of the jets of water is adjusted so as to break or cut the yarns at the surface, which cannot be envisaged when fabricating a structural or thermostructural composite material part since it is essential to preserve the integrity and the continuity of the yarns at all points within the fabric in order to avoid damaging the mechanical properties of the part. In addition, the method described in Document U.S. Pat. No. 5,217,796 leads to considerable expansion at the surface of the fabric which is penalizing for infiltrating to the core of the fabric while performing densification by CVI. Under such circumstances, the matrix becomes deposited mainly on the expanded surface of the fabric, thereby rapidly sealing the surface of the fabric and preventing the gas phase from penetrating to the core of the fabric. Having little or no matrix in the core of the fabric means that it is not possible to obtain composite material parts with mechanical properties that are satisfactory.